Intracellular signalling plays a crucial role in lymphocyte development and function. A panel of molecules involved in signalling has been described and chemical interactions between these molecules documented. However, it remains largely elusive as to whether and how the activation of an individual signalling molecule contributes to various in vivo physiological phenomena such as control of antigen receptor gene rearrangement, repertoire selection, tolerance induction, peripheral lymphocyte activation and maintenance of immune memory. Since these questions can only be properly addressed in an in vivo system, this project focuses on developing animal models by gene targeting and transgenic approaches, and using cellular, molecular and biochemical methods to analyze the impact of various altered signalling molecules on the development, function and pathology of lymphocytes in vivo. We are currently interested in understanding the regulatory functions of the protein tyrosine kinase Ctk and the adaptor molecule Grb2 of the Ras signalling pathway. For this reason, we have decided to establish animal models in which these two molecules are inactivated either in the germline or at different stages of animal development. So far, the Ctkko and Ctkfloxed chimeric mice have been obtained in our laboratory. ES cell lines carrying Grb2ko and Grb2floxed mutations were also obtained and will be used to generate mice carrying the corresponding mutations. These model systems may allow us to study in detail intracellular signalling mechanisms involved in lymphocyte development and function in a more physiological situation.